Not Applicable.
Not Applicable
This invention relates to methods and apparatus for reducing the particle size of metallic silicon to particles of a size which are subject to violent exothermic reaction with an oxidant such as oxygen, water, compounds having hydroxyl groups, or other oxidative groups.
Silicon metal is of important value in certain chemical applications as a feedstock and/or feedstock precursor in products such as glass, silicone, silanes and many other materials. In virtually all cases, the state of the silicon to be introduced includes small particle sizes, in the range of microns, narrow particle size distribution, and a surface substantially or completely free of oxygen, water, compounds having hydroxyl groups, or other oxidative groups. Providing small particle sizes of metallic silicon particulates with surfaces substantially or completely free of oxidative groups is important for improving the efficiency, yield, and quality, while reducing the costs of producing high quality optical glasses, silicon based computer chips, and other products utilizing silicon based materials.
Heretofore, the reduction of the particle size of silicon metal particulates, such as lump silicon metal, has been carried out in ball mills and the like. Ball milling involves several drawbacks. For example, the rate of milling and size of the milled product in a ball mill are a function of the diameter and speed of rotation and media (ball) size. Large media gives high rate of milling, but wide particle size distribution. Smaller media give a narrower particle size distribution but the particles are not as small. Ball milling creates heat, and as the surface area of the powder increases, it is susceptible, in the presence of water or oxygen, to undergoing a violent exothermic reaction. In ball mills, it is very difficult to keep the surface of the silicon free of oxygen or water. Further, most milling efficiency is lost if one adds a liquid to the mill. The increased potential for violent exothermic reactions between fine particle size silicon metal powder, particularly hot silicon metal powder, and oxidants, particularly oxygen, water, hydroxols, etc., has heretofore dictated the necessity for extreme precautionary measures when carrying out such a particle reduction process. These measures have included such extreme measures as the use of buildings designed to manage explosions.
Attrition mills for use in particle size reduction of silicon metal are known in the art. However, attrition mills generate even more heat than ball mills, therefore providing an explosive environment when milling of lump metallic silicon into a range of average particle sizes of silicon metal particulates that are susceptible to violent exothermic reaction with an oxidant, such as average particle sizes of 10 microns or smaller.
It is therefore an object of the present invention to provide a method for reduction of relatively large silicon metal particulates, such as lump silicon metal, to fine particle size particulates.
It is a further object of the present invention to provide an apparatus for use in the reduction of relatively large silicon metal particulates to fine particle size particulates.
It is a further object of the present invention to minimize the dangers inherent in the course of reducing relatively large silicon metal particulates to fine particle size particulates.
It is a further object of the present invention to produce fine particle size silicon metal particulates having surfaces that are substantially or completely free of oxidative groups.